article published yes Melissa A Toups author 4E099E4E-F248-11E8-B48F-1D18A9856A870000-0002-9752-7380 Beatriz Vicoso author 49E1C5C6-F248-11E8-B48F-1D18A9856A870000-0002-4579-8306 BeVi department Sex chromosomes have evolved independently multiple times, but why some are conserved for more than 100 million years whereas others turnover rapidly remains an open question. Here, we examine the homology of sex chromosomes across nine orders of insects, plus the outgroup springtails. We find that the X chromosome is likely homologous across insects and springtails; the only exception is in the Lepidoptera, which has lost the X and now has a ZZ/ZW sex-chromosome system. These results suggest the ancestral insect X chromosome has persisted for more than 450 million years—the oldest known sex chromosome to date. Further, we propose that the shrinking of gene content the dipteran X chromosome has allowed for a burst of sex-chromosome turnover that is absent from other speciose insect orders. https://research-explorer.ista.ac.at/download/14604/14618/2023_Evolution_Toups.pdf application/pdfno Oxford University Press2023 eng 1558-5646 37738212 00117034190001410.1093/evolut/qpad169 77112504-2511 https://research-explorer.ista.ac.at/record/14616 https://research-explorer.ista.ac.at/record/14617 https://git.ista.ac.at/bvicoso/veryoldx Toups, Melissa A, and Beatriz Vicoso. “The X Chromosome of Insects Likely Predates the Origin of Class Insecta.” <i>Evolution</i>. Oxford University Press, 2023. <a href="https://doi.org/10.1093/evolut/qpad169">https://doi.org/10.1093/evolut/qpad169</a>. M.A. Toups, B. Vicoso, Evolution 77 (2023) 2504–2511. Toups MA, Vicoso B. The X chromosome of insects likely predates the origin of class Insecta. <i>Evolution</i>. 2023;77(11):2504-2511. doi:<a href="https://doi.org/10.1093/evolut/qpad169">10.1093/evolut/qpad169</a> Toups, M. A., &#38; Vicoso, B. (2023). The X chromosome of insects likely predates the origin of class Insecta. <i>Evolution</i>. Oxford University Press. <a href="https://doi.org/10.1093/evolut/qpad169">https://doi.org/10.1093/evolut/qpad169</a> Toups, Melissa A., and Beatriz Vicoso. “The X Chromosome of Insects Likely Predates the Origin of Class Insecta.” <i>Evolution</i>, vol. 77, no. 11, Oxford University Press, 2023, pp. 2504–11, doi:<a href="https://doi.org/10.1093/evolut/qpad169">10.1093/evolut/qpad169</a>. Toups MA, Vicoso B. 2023. The X chromosome of insects likely predates the origin of class Insecta. Evolution. 77(11), 2504–2511. M. A. Toups and B. Vicoso, “The X chromosome of insects likely predates the origin of class Insecta,” <i>Evolution</i>, vol. 77, no. 11. Oxford University Press, pp. 2504–2511, 2023. 146042023-11-26T23:00:54Z2025-09-09T13:32:06Z